Since you're collecting the results of an iteration, this should be `staged.map` rather than .each.

I'm unclear on what the prefix modifier does here: `XMLLINT="    "`. Why not just run the command directly?

Stylistically, we put "breathing room inside the curlies", e.g.:

```ruby
staged.map { |file| ... }
```

You're also going to need to change this before it lands on master, since `staged` now returns a list of `StagedFile` instances (25caa6bfcf3a616124b77b0b65c494f59e3f4aca).